The seasonal change in day length is the major environmental factor regulating the timing of the reproductive cycle in many seasonally breeding animals. It has been clearly established that a circadian clock is involved in photoperiodic time measurement, and in mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus appears to be the location of a circadian clock(s) involved in 1) measuring the seasonal change in day length, and 2) the generation of a variety of different circadian rhythms. Presently, little is known about the neurochemical events mediating the effects of light on the circadian system and the neuroendocrine-gonadal axis. Preliminary data suggest that the neurotransmitter, acetylcholine, plays an important role in the transmission of light-dark information to the circadian and reproductive systems. Experiments in this proposal are designed to examine in detail the effects of various cholinergic and anticholinergic drugs on the circadian and reproductive systems of golden hamsters exposed to different experimental conditions. Particular attention will be focused on determining whether or not the administration of carbachol, a cholinergic agonist, accurately mimics the effects of light on both the hypothalamicpituitary-gonadal asix and the circadian rhythm of locomotor activity in male golden hamsters. Carbachol will be administered directly into specific brain regions in order to determine its site of action, and the role played by the eyes and/or the lateral geniculate nucleus in the response to carbachol will be examined. Both golden and Djungarian hamsters will be utilized in an attempt to develop an animal model in which the acute effects of light and carbachol on pituitary gonadotropin release can be compared. The long-term objective of this research program is to elucidate the complex sequence of neural and endocrine events which occur following changes in the light-dark cycle. The proposed studies represent an important step in achieving this goal. In addition, the results of the proposed experiments are anticipated to yield new insights into the physiology of both the circadian and reproductive systems and should aid in the understanding of various disorders that are related to dysfunction of the circadian and reproductive systems in humans.